The invention generally relates to mobile wireless communication systems. In particular, the invention relates to cell search in such systems.
FIG. 1 illustrates a mobile wireless communication system. The communication system has a plurality of base stations 121–12n (12). Each base station 12 communicates with user equipments (UEs) 141–14n (14) within its operating area or cell 161–16n (16). When a UE 14 is first activated, it is unaware of its location and with which base station 12 (or cell 14) to communicate. The process where the UE 14 determines the cell 14 to communicate with is referred to as “cell search.”
In typical code division multiple access (CDMA) communication systems, a multi-step process is used for cell search. For step one, each base station 12 transmits the same primary synchronization code (PSC) in a primary synchronization channel (PSCH) 18. In a frequency division duplex (FDD) communication system using CDMA, the PSCH 18 is all the timeslots of a frame, such as fifteen (15) timeslots as shown in FIG. 2. Each base station's transmitted PSC is sent in all the timeslots.
In a time division duplex (TDD) communication system using CDMA, the PSCH is one timeslot out of fifteen for type I cell search (as shown in FIG. 3a), such as slot 0 or in general K, where 0≦K≦14, or two timeslots for type II cell search (as shown in FIG. 3b), such as slots 0 and 8 or in general K, where 0≦K≦6, and K+8. Each base station transmits the same PSC in the PSCH timeslot(s). To reduce interference between secondary synchronization codes (SSCs) used in step two, each PSC is transmitted at a different time offset. The PSC offsets are at a set number of chips.
For both FDD/CDMA and TDD/CDMA, the UE 14 determines the base station 12 to synchronize to by searching the PSCH for received PSCs, such as using a matched filter. An example of the results of such a search in a TDD system is shown in FIG. 4. As shown in FIG. 4, peaks 261–262 occur in the PSCH where there is a high correlation with the PSC code. Typically, the search results are accumulated over multiple frames to improve accuracy. Using the accumulated results, the PSC peak locations are determined in the PSCH.
Along with each base station's transmitted PSC, each base station 12 also simultaneously transmits secondary synchronization codes (SSCs) for both FDD and TDD type I and type II. The SSCs sent by each base station 14 are used to identify certain cell parameters, such as the code group and PSC time offset used by the cell. The UE 14 typically uses a correlator to detect the SSCs and the data modulated on them at each PSC peak identified in step I. In step III, the UE 14 completes the synchronization to one of the detected base stations 12 using the information gathered in steps I and II. In step III for FDD, typically, the UE 14 match filters the common pilot channel (CPICH) to identify the cell specific scrambling code to allow the UE 14 to read the broadcast control channel (BCCH). In TDD step III for both types I and II, typically, the UE 14 detects the cell specific midamble used in the broadcast channel and subsequently reads the broadcast channel.
This approach to cell search has drawbacks. One drawback is the memory required to store a frame's worth of input signal and PSC correlation values. Storing all these data points uses valuable memory resources. Another drawback is processing a frame's worth of data requires considerable processing time. Finally, storing only peak locations ignores other valuable information gathered during the correlation, such as the peak's shape.
Accordingly, it is desirable to have alternate approaches for cell search.